1. Field of the Invention
The present invention relates to glycol/water separation. More particularly, the invention relates to a process and a system for recovering glycol from glycol/brine streams produced from oil or natural gas wells.
2. Description of the Related Art
A common problem associated with natural gas production is the formation of hydrates. Hydrates are solid compounds that form as crystals and resemble snow in appearance. They are created by a reaction of natural gas with water, and when formed, they are about 10% hydrocarbon and about 90% water. To prevent plugging of production lines and equipment by hydrates, it is common to inject a hydrate inhibitor into the gas well.
Traditionally, when producing gas offshore, methanol has been used as a hydrate inhibitor because it lowered the freezing point of water vapor and thus prevented hydrate formation in flow lines. The methanol was produced from the well along with the brine and the methanol/brine solution was often disposed of by dumping it into the ocean.
More recently, economic and environmental considerations have forced offshore hydrocarbon producers to consider techniques for recovering hydrate inhibitors from the inhibitor/brine streams. Processes that recover methanol from methanol/brine streams are known to those skilled in the art, however, there are disadvantages to these processes. Particularly, methanol recovery systems generally leave a large portion of the methanol in the brine stream that is lost during disposal. Therefore, the environmental problems associated with disposal of the brine stream continue to exist. Additionally, some methanol is lost along with the vapor phase. Because methanol is lost in the recovery process, additional methanol must be purchased and transported to the offshore platform to make up for the losses.
It has been known to use glycol as a hydrate inhibitor for natural gas streams containing fresh water vapor. Glycol recovery systems are also known to those skilled in the art to remove glycol from the glycol/water streams. Generally, these systems are designed to produce glycol/water streams having between about fifty (50%) and about ninety-five (95%) percent glycol.
As shown in FIG. 1, prior art glycol recover systems primarily consisted of a distillation column 10 in which the glycol was concentrated by distilling off the accompanying water. A natural gas stream containing glycol and water was introduced into a series of separator vessels 12 and 14 where the pressure was reduced to flash off the natural gas. The glycol/water stream was then introduced into distillation column 10 where it was heated by reboiler 16, typically a steam reboiler, to drive the water overhead and concentrate the glycol. The recovered glycol stream produced by this process was approximately ninety-nine percent (99%) glycol.
While glycol is an effective hydrate inhibitor for use with natural gas wells, the glycol recovery systems of the prior art are not particularly suited for recovering glycol from glycol/brine solutions produced from the natural gas wells.
One significant problem with the prior art system of FIG. 1 is created by the salt and other solids contained in the glycol/brine streams. Glycol/brine streams produced from natural gas wells typically contain between about forty percent (40%) and about sixty percent (60%) glycol, about sixty percent (60%) and about forty percent (40%) water with about ten percent (10%) to about twenty-five percent (25%) weight percent dissolved salt in the produced water. The distillation process often results in precipitation of the salt that can foul and plug the recovery system.
Additionally, the prior art glycol recovery systems such as shown in FIG. 1 are extremely energy intensive. Distillation column 10 requires a reboiler 16 to provide the heat necessary to drive off the water vapor. The heat duty required by the reboiler 16 is significant, approximately 300 MM BTU's per hour for a nominal 5,000 barrels per day ("BPD") glycol recovery unit.
Thus, the need exists for an environmentally safe and energy efficient process for recovering hydrate inhibitors that are produced from oil or natural gas wells along with a brine stream. Particularly, the need exists for a process that recovers glycol from glycol/brine streams produced from oil or natural gas wells that is less energy intensive than the prior art systems and is not subject to fouling or plugging problems caused by salt and other solids in the stream. Additionally, the need exists for such a glycol recovery system that can be used on offshore production platforms.